A good deal of research effort within the last decade has been conducted in correlating changes in permeability across bilayers where specific perturbations to the structure of the bilayers can be responsible for changes in permeability (from an "off" position to an "on," or "open," position). A leading researcher in the area has been Yoshio Okahata.
In the article "Lipid Bilayer-Corked Capsule Membranes, Reversible, Signal-Receptive Permeation Control," Acc. Chem. Res., 19, pp. 57-63 (1986), Okahata describes capsules formed of porous nylon. The nylon forms a capsule with an inner layer having pores on the order of 1-2 nm in diameter, while an outer layer has pores on the order of 100-300 nm in diameter. These pores are filled, or corked, with multilamellar bilayers such as 2C.sub.12 PO.sup.-.sub.4. Okahata reported permeation enhancements under 60-V fields (60 kV cm.sup.31 1 since the capsule membrane is 1 .mu.m thick). However, the corked capsules permitted slow permeation of NaCl, even in the absence of a field, at the rate of 5.5.times.10.sup.-7 cm.sup.2 s.sup.-1, which slow "off" permeation rate increased slightly after a 3 minute charging duration to 5.7.times.10.sup.-7 cm.sup.2 s.sup.-1.
Okahata et al., "The Electrical Breakdown and Permeability Control of a Bilayer-Corked Capsule Membrane in an External Electric Field," J. Am. Chem. Soc., 108, pp. 2863-2869 (1986), continues earlier studies with variations in the head groups for the bilayer-forming amphiphile.
Okahata and En-na, "Permeability-Controllable Membranes . . . Electrochemical Responsive Gate Membranes of a Multibilayer Film Containing a Viologen Group as Redox Sites," J. Phys. Chem., 92, pp. 4546-4551 (1988), report permeability rates of 3.0.times.10.sup.-7 cm.sup.2 s.sup.31 1 in an oxidized "off" form and permeability decreased by a factor of 5 (6.3.times.10.sup.-8 cm.sup.2 s.sup.-1) for a reduced "on" form.
Okahata and Takenouchi, "Permeability Controllable Membranes . . . Electrochemical Redox-Sensitive Gate Membranes of Polypeptide Films Having Ferrocene Groups in the Side Chains," Macromolecules, 22, pp. 308-315 (1989), describe synthetic polypeptide membranes that were cast on a platinum minigrid sheet. These films contained thiol groups that could be oxidized and reduced by electrochemical potentials. Permeation enhancement up to 6 times were reported, with permeability rates of the reduced form being about 5-12 .times.10.sup.-8 cm.sup.2 s.sup.-1.
Okahata and Shimizu, "Preparation of Bilayer-Intercalated Clay Films and Permeation Control Responding to Temperature, Electric Field, and Ambient pH Changes," Langmuir, 5, pp. 954-959 (1989), describe the preparation of a self-standing bilayer-intercalated clay film. These bilayer-intercalated clay films were prepared by admixing an aqueous colloidal solution of clay (montmorillonite) and an aqueous dispersion of ammonium amphiphiles. Precipitates were dried as powder and exchangeable cations in the interlayer of the clay were changed for cationic bilayer-forming amphiphiles. A self-standing film was cast on a polyester minigrid (50 .mu./270 mesh, 70 .mu.m thick). The film thickness was about 100 .mu.m. X-ray analyses indicated film microstructure as being stacked clay layers, each separated by amphiphile forming bilayer structures in the clay interlayers. In the absence of electric fields, permeation was 1.2.times.10.sup.-9 cm.sup.2 s.sup.-1 but with a DC applied voltage of 2.5 V (50 V/cm), permeability was enhanced.
These Okahata references all describe macroscopic assemblies utilizing bilayers, or multiples of bilayers, with the best "off" position permeability value being in the 10.sup.-9 cm.sup.2 s.sup.-1 range. However, composite structures with better "off" position permeability and highly controllable, reproducible permeability in fabricating structures are desirable.
Devices using amphoteric molecules or Langmuir-Blodgett film forming techniques have been recently described for various applications. Thus, U.S. Pat. No. 5,234,566, issued Aug. 10, 1993, inventors Osman et al., describes a biosensor having a lipid membrane. The membrane includes a gated ion channel. The membrane has a closely packed array of self-assembly amphiphilic molecules. In one form of the biosensor, the conductance of the ion channels is dependent on an electric field applied across the membrane. By "gated ion channel" is meant that the passage of ions therethrough is dependent on the presence of an analyte. The amphiphilic molecules are preferably cross-linked. Attachment of analyte to a receptor moiety attached or associated with the ion channel causes the receptor to change into a state where ions may pass through the ion channel.
Another recent device is described by U.S. Pat. No. 5,293,261, issued Mar. 8, 1994, inventors Shashidhar et al. This liquid crystal device has a Langmuir-Blodgett film of a liquid crystal polymer on a substrate between a pair of electrodes. The Langmuir-Blodgett film is said to undergo electric field-induced ferroelectric switching. The exemplary liquid crystal polymers of the film have poly(meth)acrylic, poly(meth)acrylate, and polysiloxane backbones, and the films are formed by dipping with five to fifty layers of LCP preferred.
A need exists for assemblies that are sensitive and selective for various molecular release or capture applications, but which are readily and reproducibly fabricated.